It is a common task on construction sites to use devices for detecting underground utility lines, structures, or services before or while earth-moving. Such utility lines are often occurring in form of services for supplying electricity, gas, fuel, water, sewage or communication data, etc. Although the location of most of these utility lines is—or at least should be—already known from a surveyor's plan, their locations can have uncertainties or there could be additional unmentioned utility lines. It is also a problem that there are wide range of sources for getting such surveyor's plans, for example multiple different supply companies, each compiling their own data, probably in a proprietary data format. Often underground utility lines are also simply overlooked or wrongly assessed. Avoidance of damage to underground utility lines while digging in a trench or in areas being excavated is an important task. Damage to a utility line can cause serious impact and costs. Therefore, measures are taken or are even statutory, to detect underground utility lines, preferably evaluating the utility location and/or utility depth of such an underground wiring, conduit or pipe. Devices for this purpose are known as Cable Detection Tools or Cable Avoidance Tools—also called CAT. Such a detection device is mostly embodied movable, preferably designed and built as a handheld device to be carried around by a worker. In special movable embodiment, it can also be mounted at a bucket of an excavator and being move with the bucket.
The classical way to locate underground utility lines is to detect electromagnetic fields sent out by the utility line itself. To do so, the utility line requires having a naturally occurring electrical signal, which emits an electromagnetic field that is detectable above the ground, such as e.g. a live power supply line. Power-lines commonly provide currents with a fundamental frequency of e.g. 50 Hz or 60 Hz, but also harmonics of the fundamental frequencies can be used for detection purposes, in particular zero sequence harmonics. For other services like communication lines, etc. there are also other specific frequency bands which are evaluated by such devices. An embodiment of such a device is for example described in EP 2 362 241. To detect other types of utility lines without naturally occurring signals, for example a wiring system of switched off street lights, unused or low-voltage communication cables, gas- or water-pipes, those can e.g. be directly connected to a signal-generator (e.g. U.S. Pat. No. 4,438,401) or an AC current can be conducted to them via soil (e.g. EP 9 166 139 or EP 2 645 133). In hollow pipes like gas or water pipes, a conductor or a transmitting sonde can be introduced (e.g. U.S. Pat. No. 5,194,812).
The detection itself can be carried out by moving the detection device across a suspected area, e.g. by walking and/or swinging the device side to side. The location where the strongest (or dependent on the setup also weakest) signal occurs can then be marked. A depth value can be determined according a difference of the detected signals at two detectors which are spaced apart in a known distance in the device. By using more than two detection coils in the device, additional location information can be derived, if the detectors are well calibrated. Therefore, upper-class devices comprise at least two sets of multiple detection coils each. For example, U.S. Pat. No. 8,248,056 shows a detection device with a pair of spaced-apart 3D magnetic sensor arrays and an intermediate pair of sensors for a detection according to tensors-calculations of the picked up field from the utility line.
US 2005/156,600 shows an omnidirectional sonde and line locator having a sensor array including three mutually orthogonal antennas which are sharing a common centre point. To be able to establish such detection, it is mentioned that the coils are calibrated with respect to the other two coils of the same array, which is done by placing the system within a tubular solenoid field in a therefore dedicated calibration camber. U.S. Pat. No. 8,635,043 or EP 1 843 177 are describing such factory calibration rigs, in which an individual fine tuning of each cable detection device can be determined in a factory or laboratory environment.
In U.S. Pat. No. 7,733,077 a portable locator with an articulated antenna node configuration is shown, comprising sets of orthogonally arranged antennas. For finding and mapping buried objects, it uses Doppler-Radar and GPS navigation. It uses a calibration by a large Helmholtz coil which is provided as a dedicated active calibration coil, wherefore a finite element calculation or a special calibration frame setup with multiple small coils in a specific, well defined configuration is suggested. EP 3 002 614 (herewith incorporated by reference) also considers a CAT-Tool calibration.
The detection results can then be presented to the operator, preferably in graphical form by a 2D map-like computer generated graphic showing detected utility lines and their respective depths.
It is common practice to trench multiple different utility lines substantially side by side in close proximity. Thereby the location and identification of a specific utility can get complicated. Also, there can be many uncertainties and influences. Another operator might achieve other detection results, as he uses different approaches, strategies and detection modes, dependent on his skills.